How to create an advertising image flying in the web page?

I have saw from some web site( ex. www.china.com) small advertising images that can move around in the web page. These small images can just fly like a butterfly within the web page, and when you click on it, the advertised links will be opened. Does anybody know how to do this in DHTML and JavaScript? If possible, I would appreciate some sample code as well. Thanks in advance.

liang,
this is the javascript they used for that( rt click and click view source)

<!--
document.ns = navigator.appName == "Netscape"
//--
var rnumx1=new Array();
var rnumx2;
var rnumx3;
rnumxtemp="";
for(i=0;i<3;i++){
rnumx2 =Math.round(Math.random()*10);
rnumx2!=10 ? rnumx3=rnumx2:rnumx3=9;
//document.write("["+rnumx3+"]");
rnumx1[i]=rnumx3;
if (rnumx1[0]>4||rnumx1[0]<1){
rnumx1[0]=1;
}
if (rnumx1[1]>2&&rnumx1[0]==4){
rnumx1[1]=1;
}
rnumxtemp+=new String(rnumx1[i]);
}
//--
window.screen.width>800 ? imgheight=150:imgheight=rnumxtemp
window.screen.width>800 ? imgright=20:imgright=20
window.screen.width>800 ? imgleft=20:imgleft=20
function threenineload()
{
if (navigator.appName == "Netscape")
{
if(document.getElementById) {
document.getElementById('threenine').pageY=pageYOffset+window.innerHeight-imgheight;
document.getElementById('threenine').pageX=imgright;
document.getElementById('threenine1').pageY=pageYOffset+window.innerHeight-imgheight;
document.getElementById('threenine1').pageX=imgleft;
} else {
document.threenine.pageY=pageYOffset+window.innerHeight-imgheight;
document.threenine.pageX=imgright;
document.threenine1.pageY=pageYOffset+window.innerHeight-imgheight;
document.threenine1.pageX=imgleft;
}
threeninemove();
}
else
{
threenine.style.top=document.body.scrollTop+document.body.offsetHeight-imgheight;
threenine.style.right=imgright;
threenine1.style.top=document.body.scrollTop+document.body.offsetHeight-imgheight;
threenine1.style.left=imgleft;
threeninemove();
}
}
function threeninemove()
{
if(document.ns)
{
if(document.getElementById) {
document.getElementById('threenine').style.top=pageYOffset+window.innerHeight-imgheight
document.getElementById('threenine').style.right=imgright;
document.getElementById('threenine1').style.top=pageYOffset+window.innerHeight-imgheight
document.getElementById('threenine1').style.left=imgleft;
} else {
document.threenine.top=pageYOffset+window.innerHeight-imgheight
document.threenine.right=imgright;
document.threenine1.top=pageYOffset+window.innerHeight-imgheight
document.threenine1.left=imgleft;
}
setTimeout("threeninemove();",40)
}
else
{
threenine.style.top=document.body.scrollTop+document.body.offsetHeight-imgheight;
threenine.style.right=imgright;
threenine1.style.top=document.body.scrollTop+document.body.offsetHeight-imgheight;
threenine1.style.left=imgleft;
setTimeout("threeninemove();",80)
}
}
function MM_reloadPage(init) { //reloads the window if Nav4 resized
if (init==true) with (navigator) {if ((appName=="Netscape")&&(parseInt(appVersion)==4)) {
document.MM_pgW=innerWidth; document.MM_pgH=innerHeight; onresize=MM_reloadPage; }}
else if (innerWidth!=document.MM_pgW || innerHeight!=document.MM_pgH) location.reload();
}
MM_reloadPage(true)
{
document.write("<div id=threenine1 style='position: absolute;width:64;top:200;visibility: visible;z-index: 1'><a href=http://ads.china.com/globeedu.html target=_blank><img src=http://ads.china.com/globeedu_icon.gif border=0 width=80 height=80></a></div>");
document.write("<div id=threenine style='position: absolute;width:64;top:200;visibility: visible;z-index: 1'><object classid='clsid

27CDB6E-AE6D-11cf-96B8-444553540000' codebase='http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=5,0,0,0' width='80' height='80'><param name=movie value='http://ads.china.com/mercan/80_80.swf'><param name=quality value=high><embed src='http://ads.china.com/mercan/80_80.swf' quality=high pluginspage='http://www.macromedia.com/shockwave/download/index.cgi?P1_Prod_Version=ShockwaveFlash' type='application/x-shockwave-flash' width='80' height='80'></embed></object></div>");
threenineload()
}
//-->
</script>

Look at http://www.dynamicdrive.com they have all scripts that deal with this matter. It is a rather easy thing to do in HTML and I have a couple of these scripts hiding on my harddrive somewhere.
Eric